Multimode cavity resonator with triangular coupling holes



Oct. 21, 1969 J. R. WHITE 3,474,212

MULTIMODE CAVITY RESONATOR WITH TRIANGULAR COUPLING HOLES Filed Oct. 13, 1967 2 Sheets-Sheet l MICROWAVE SOURCE INVENTOR.

JEROME R. WHITE BY Maw/942 i ATTORNEY Oct. 21, 1969 J. R. WHITE 3,474,212

MULTIMODE CAVITY RESONATOR WITH TRIANGULAR COUPLING HOLES Filed Oct. 13, 1967 2 Sheets-Sheet 2 I v 3 FIG.2 W I l5 l8 FIG.4 54 |o2 INVENTOR.

JEROME R.WH|TE BY W%azz- ATTORNEY United States Patent US. Cl. 21910.55 11 Claims ABSTRACT OF THE DISCLOSURE A rectangular multimode cavity resonator is excited by microwave energy introduced into the resonator through a triangular coupling hole located at a corner of one of the boundary walls forming the rectangular cavity resonator. The triangular coupling hole is located with its apex at the corner of the wall. A dominant TE mode waveguide couples the cavity resonator at the coupling hole to a microwave energy source. The waveguide is orientated relative to the coupling hole so that the electric field component of the electromagnetic field propagated by the waveguide bisects the apex of the coupling hole. Mechanical mode stirrers are mounted within the cavity resonator and coupled to a suitable drive motor located exteriorly of the cavity resonator. Supporting structure is provided within the cavity resonator to support the material being heated at a distance of about 2 /2)\ from the nearest boundary wall.

BACKGROUND OF INVENTION Heating materials with microwave energy has become common in a number of industrial applications. Generally, one of the most important objects in designing microwave heating systems is to construct the system so that uniform heating is produced in the material independent of its size and shape. In certain applications, the energy of the electromagnetic field should be distributed in a particular non-uniform manner in the zone provided for heating, while in others, the energy should be distributed uniformly in the heating zone. An improper microwave energy distribution creates localized regions of maximum and minimum heating in the materials. In most cases, such non-uniform heating is undesirable and often is harmful to the materials. Unfortunately, the inability to control the microwave energy distribution conveniently and properly has limited the extent of use of microwave energy in industrial heating applications.

Multimode microwave cavity and waveguide resonators are susceptible to being excited in a plurality of electromagnetic field mode patterns, each occurring at a different frequency. All of the mode patterns of a particular microwave resonator may be classified into three distinct subsets of resonator modes. The resonator modes of each subset are distinguishable fromthose of the others by the orientations of their electromagnetic fields and associated wall currents. For a given microwave source frequency and microwave coupling means, a multimode resonator may be excited in a mode pattern whose frequency is close to the source frequency. The bandwidth of the mode determines how close the frequency of the source must be to the mode frequency to excite the resonator in the particular mode. Heretofore, mode stirrers have been employed in multimode microwave resonators in an attempt to obtain a particular, usually time-averaged uniform, electromagnetic field, hence, microwave energy distribution throughout the zone provided for heating materials. Operation of mode stirrers causes the electrical space as seen by the electromagnetic field in the resonator to change. This change in the electrical space shifts the 3,474,212 Patented Oct. 21, 1969 ice frequencies at which the various mode patterns occur. By shifting the mode pattern frequencies so that they sequentially and cyclically coincide with the frequency of the microwave energy source, the distribution of the electromagnetic field, hence, microwave energy in the resonator is caused to change cyclically.

The time-averaged distribution of the energy depends upon the number of difierent mode patterns coupled to within the resonator and the amount of energy delivered to each of the different mode patterns excited in the resonator. For example, to obtain a time-averaged uniform distribution of microwave energy throughout a heating zone having dimensions on the order of multiples of A, the free-space wavelength of the applied energy, it is preferred to deliver energy uniformly to a large number of the possible resonator modes. However, the prior art techniques commonly employed in delivering microwave energy to the resonator provide coupling to only a few of the possible resonator modes. As a consequence of coupling to only a few of the possible resonator modes, either the uniformity of the time-averaged energy distribution is enhanced only slightly or the undesirable nonuniformity is intensified due to re-enforcing mode patterns being excited within the resonator. Furthermore, when only a few of the possible resonator modes are excited in the resonator, the ability to establish particular nonuniform time-averaged microwave energy distributions is curtailed.

By cyclically energizing a number of resonator modes belonging to all of the resonator mode subsets, the localized field regions throughout the resonator can be timeaveraged to provide a desired net electromagnetic field, hence, microwave energy distribution therein. To couple to a large number of resonator modes belonging to all three subsets, microwave energy must be delivered to the resonator in the form of electromagnetic fields which have components of electric field orientated in the directions of the wall currents which would be characteristic of each of the possible sets of resonator modes at the point at which energy is introduced into the resonator. In an article by Paul W. Crapuchetts, Microwaves On The Production Line, Electronics, Mar. 7, 1966, pp. 123430, it is suggested that a time-averaged uniform electromagnetic field distribution for heating materials can be realized by exciting a multimode cavity resonator with microwaves coupled thereto by a coupling loop located in the cavity at the junction of three intersecting cavity boundary walls. However, coupling loops have certain limitations and disadvantages associated therewith that render them less attractive than other microwave guide structures such as single conductor waveguides. Specifical- 1y, microwave transmission systems employing coupling loops have limited power handling capabilities. Furthermore, very complicated structures are required if the coupling loop system must be cooled. In addition coupling loops tend to spark and become dirty thereby requiring cleaning which often is diflicult to accomplish unless the system is disassembled.

SUMMARY OF INVENTION through a wall plane thereof microwave energy received from a single conductor waveguide structure joining the resonator to a microwave energy source. The waveguide is orientated relative to the corner so that the electromagnetic field propagated thereby has its electric field component in a direction of a line that divides the angle defined by the wall corner.

Energizing the various modes is accomplished by also providing means for shifting the frequencies at which the resonator modes occur so that the mode pattern frequencies sequentially and cyclically coincide with the source frequency. As the mode pattern frequency of each of the resonator modes coincides with the source frequency, energy is delivered to that mode from the waveguide associated with the coupling hole.

Accordingly, it is an object of this invention to provide a microwave heating system for heating materials.

More particularly, it is an object of this invention to provide a microwave heating system which presents a time-averaged electromagnetic field distribution for uniformly heating materials.

Another object of this invention is to provide a multimode resonator type of microwave heating system where in the resonator is excitable in resonator modes belonging to all of the subsets of resonator modes.

Still another object of this invention is to provide a microwave heating system which presents a time-averaged uniform electromagnetic field distribution for heating materials.

Yet another object of this invention is to provide a multimode resonator type of microwave heating system wherein the electromagnetic field mode pattern is varied cyclically through resonator modes belonging to all of the subsets of resonator mode subsets.

It is a further object of this invention to provide a multimode resonator type of microwave heating system wherein a number of reasonator modes belonging to all of the subjects of resonator modes are uniformly excited.

It is still a further object of this invention to provide a multimode resonator type microwave heating system wherein a number of resonator modes belonging to all of the subsets of resonator modes are excited at one location of the resonator structure.

It is yet another object of the present invention to provide a microwave heating system which presents a desired time-averaged electromagnetic field distribution for heating materials which does not have the limitations and disadvantages associated with such systems employing coupling loops to deliver the microwave energy to the apparatus in which the materials are heated.

BRIEF DESCRIPTION OF DRAWINGS The foregoing and other objects and advantages of the present invention will become more apparent from the following detailed description and appended claims considered together with the accompanying drawing in which:

FIGURE 1 is a perspective view of one embodiment of a microwave heating system in accordance with the present invention,

FIGURE 2 is an end sectional view of the microwave heating system taken along line 22 of FIGURE 1.

FIGURE 3a-c schematically illustrate the orientations of the conduction currents in the boundary walls of a resonator at a junction of three intersecting boundary walls at an instant for each of the three subsets of resonator modes.

FIGURE 4 is a fragmentary top view of one corner of the microwave resonator of the heating system taken along line 44 of FIGURE 1.

FIGURE 5 is an enlarged exploded perspective view of the waveguide microwave guide and mounting structure delineated by line 55 of FIGURE 2.

4 DESCRIPTION OF THE PREFERRED EMBODIMENT With reference to FIGURES l and 2, a conveyorized multimode rectangular cavity resonator embodiment of the microwave heating system 11 of the present invention is illustrated. The microwave heating system 11 includes a rectangular type multimode cavity resonator 12 of a size sufficient to support a plurality of modes of electromagnetic waves at the operating frequency. The cavity resonator 12 is constructed of conductive material, such as aluminum, and has a plurality of planar boundary walls including end walls 13 and 14, side walls 16 and 17, and top and bottom walls 18 and 19 respectively secured together, as by welding, to define enclosure 21. The height, length and width dimensions of the enclosure 21 are made large compared to I\, the free space wavelength of the applied energy, for example, approximately 7A, 17 )t, and 8% respectively.

Products to be halted by microwave energy delivered to the cavity resonator 12 are transported through the resonator by a moving conveyor belt 22 constructed of microwave transparent material, such as cotton cloth, Teflon or polypropylene. To facilitate the circulation of air around the products, the conveyor belt 22 is perforated to define air passages 23 therethrough. The conveyor belt 22 enters the cavity resonator 12 from its feed end 24 through a first microwave absorbing end trap 25 locatedat the end wall 13. The conveyor belt 22 leaves the cavity resonator 12 at its discharge end 26 through a second microwave absorbing end trap 27 located at the end wall 14. The end traps serve to prevent the escape of hazardous microwave energy from the cavity resonator 12 while allowing access thereto during operation.

Each of the absorbing end traps 25 and 27 includes a rectangular aluminum box 28 surrounding an annularlike container in the form of a tubular member 29 for holding lossy material, preferably such as water, ethylene glycol, glycerol or any low molecular weight monohydric alcohol. The tubular member 29 defines a product tunnel 31 of rectangular cross section through which the conveyor belt 22 passes to transport products through the enclosure 21. The end traps 25 and 27 are mounted to the end walls 13 and 14 respectively, for example, by welding, with their respective product tunnels 31 aligned with passageways 32 defined in each of the end walls 13 and 14 For non-hazardous operation, the amount of microwave energy escaping to the surroundings of the microwave heating device should be maintained below the accepted standard of 10 milliwatts/(centimeter) (mw./cm. With 5 kilowatts (kw.) of power at 2450 me. applied to the cavity resonator 12, the microwave energy escaping to the surroundings will be maintained substantially below the level of 10 mw./cm. by constructing the end traps 25 and 27 to have dimensions of at least 6% long, 5)\ wide and 5x high with the tubular member 29 having a wall thickness of 3M 2.

To provide ease of access to enclosure 21 in order to, for example, facilitate the performance of maintenance, the side wall 17 is provided with two conductive access doors 33 longitudinally spaced in the direction of the conveyor belt travel. Each door 33 is hinged at 34 and is provided with a pivotally mounted bar handle 36 for opening and closing the door. A compressable conductive V-shaped rib structure 37 is mounted circumadjacent about each door opening 38 to prevent hazardous microwave energy from escaping between the doors 33 and the side wall 17. Each door 33 is forceably held against the rib structure 37 by the engagement of the bar handle 36 in the bar receiving member 39.

To enhance the microwave heating operations, an aluminum air plenum 41 communicating with an air compressor (not shown) delivers an air flow to the enclosure 21 through aluminum open-ended air ducts 42 spaced longitudinally in the direction of the conveyor belt travel. The air ducts 42 are secured by welding one of their ends to the air plenum 41. The air plenum 41 and ducts 42 are secured at side wall 16 by welding the opposite ends of the air ducts 42 thereto at locations above the conveyor belt 22. To convey away the circulated air, an aluminum exhaust plenum 43 receives the circulated air from the enclosure 21 through aluminum open-ended exhaust ducts 44. The exhaust ducts 44 are secured by welding one of their ends to side wall 16 at locations below the conveyor belt 22 longitudinally spaced in the direction of conveyor belt travel. The exhaust plenum 43 is fastened by welding to the other ends of the exhaust ducts 44.

To prevent the escape of microwave energy through the air and exhaust ducts 42 and 44, the ducts are constructed to have a length of at least three times their diameter and cross sectional dimensions so that the free space cutoif wavelength relative to the highest frequency significant energy mode excited in the cavity resonator 12 is significantly less than the free space wavelength of that mode. With the dimensions of the ducts 42 and 44 adjusted in accordance with these limitations, the ducts 42 and 44 will not support the free transmission of electromagnetic fields at hazardous or undesirable microwave energy levels.

Pursuant to the present invention, microwave energy is delivered to excite the cavity resonator 12 through at least one particularly located coupling hole 51. The coupling hole 51 is defined by one of the resonator boundary walls at a corner thereof adjacent any of the junctions defined by three intersecting resonator boundary walls. Microwave energy is introduced into the cavity resonator 12 from a single conductor waveguide structure, for example, rectangular waveguide 52. The waveguide 52 is excited and orientated relative to the Wall corner at which the coupling hole 51 is located so that, at the coupling hole 51, the electromagnetic field propagated by the waveguide has its electric field component in a direction which divides, preferably, by bisecting the angle 9 defined by the wall corner.

The manner in which energy is delivered from the waveguide 52 to the resonator modes of all three subsets through the particularly located coupling hole 51 is best understood with reference to FIGURES 3oc-c. Each of the FIGURES 3a-c schematically depict the pattern of wall currents at a particular instant which are characteristic of the three resonator mode subsets in the vicinity of the junctions formed by three intersecting resonator boundary walls, such as, junction 56 formed by the intersecting end wall 13, side Wall 17 and top wall 18 of the cavity resonator illustrated in FIGURES 1 and 2. The wall current pattern for each of the possible resonator modes will be the same as one of those represented by arrows 57, 58 and 59 in one of the FIGURES 3ac. At a given source frequency, the wall currents of any resonator mode which at any instant intersects the edges of the boundary walls, e.g., 13, 17 and 18, converging to the corners 61, 62 and 63 will be of the same sense along the edge lengths between each of the corners and points 64 located at distances \/2 therefrom. Furthermore, the sense of the instantaneous wall currents will reverse at positions 66 located at perpendicular distances of 7\/4 or greater from the edges of the boundary walls. In addition, the wall currents will be zero at the corners of the boundary walls for any of the possible resonator modes. Consequently, instantaneous wall currents of opposite senses will never simultaneously intersect the edges of the boundary walls along the lengths thereof up to 7\/2 from their corners, and will never exist simultaneously in the portions of the boundary walls up to M4 from the wall edges.

Therefore, since coupling will exist between a waveguide structure and the resonator modes of a subset when electromagnetic fields propagated by the waveguide includes components of electric field which are parallel to the wall currents characteristic of the subset at the location at which the waveguide is coupled to the resonator, resonator modes belonging to all three of the resonator mode subsets can be excited in the resonator 12 by coupling such waveguides thereto through the coupling hole 51 located at a corner of the resonator boundary walls at one of the junctions. By confining the coupling hole 51 to a triangular segment 53 (see FIG. 4) of a wall bounded by, for example, the converging edges 54 and 55 (see FIG. 4) of the top wall 18 defining the angle 0 at wall corner 63 and a line connecting points along the edges 54 and 55 of the top wall 18 at a distance of M2 from the corner 63, a waveguide excited to propagate either TE or TM modes can be joined to the cavity resonator so that the instantaneous field in the guide will not simultaneously include components of electric field which are parallel and antiparallel to the instantaneous wall currents which would be characteristic of the possible resonator modes at the location of the coupling holes if a boundary wall portion existed in its place. This facilitates uniform energy coupling to the three subsets of resonator modes. However, some degree of simultaneous parallelism and antiparallelism might be desirable, for example, when a particular non-uniform energy distribution is wanted. Equal amounts of simultaneous parallelism and antiparallelism is to be avoided when energy transfer between the guide and resonator is desired, because when such conclitions exist, very little if any energy is transferred between the resonator and guide structure. To insure coupling to all of the possible resonator modes, the coupling hole should not be farther than \/2 /2 from the wall corner formed by the intersecting edges of the boundary wall at which the coupling hole 51 is located, should extend over the triangular wall segment defined by points less than A along the boundary wall edges and a line connecting those points, and should have greater area portions in triangular segment 53 of the boundary wall than outside the segment.

Selective coupling to the three resonator mode subsets can be achieved with a single waveguide structure propagating either TE or TM waves. A waveguide propagating TE waves and coupled to the resonator 12 through, for example, a corner of the top wall 18 with the electric field of the propagated in the waveguide oriented in the direction of a line bisecting the angle 0 defined at the wall corner will have components of its electric field which are parallel to the direction of the wall currents 57 of FIGURE 3a, the wall currents 58 of FIG- URE 3b, and the wall currents 59 of FIGURE 30, respectively characteristic of resonator modes belonging to each of the subsets at the corner of the top wall 18. Because of this parallel relationship between the electric field components of the field propagated in the waveguide and the wall currents characteristic of each of the resonator mode subsets, energy can be delivered to resonator modes belonging to all three subsets from the waveguide propagating 'IE mode energy.

A waveguide propagating TM waves also can achieve coupling to the resonator modes belonging to each of the three subsets. The TM waveguide would be located relative to the wall corner in a manner similar as a TE waveguide, i.e., with electric field of the field propagated in the TM waveguide in the direction of a line bisecting the angle 0 defined at the wall corner.

In operation, when the frequencies of the resonator modes belonging to the subset depicted in 'FIGURE 3a coincide with the source frequency, energy will be delivered to those resonator modes from the waveguide 52. When the frequencies of the resonator modes belonging to the subset depicted in FIGURE 3b coincide with the source frequency, energy also will be delivered from the waveguide 52 to those resonator modes. Energy from waveguide 52 will be delivered to the resonator modes of 7 the subset depicted in FIGURE 30 when their frequencies conicide with the source frequency.

Referring now to FIGURE 1, 2 and 4, one embodiment of the present invention has a triangular shaped coupling hole 51 at the wall corner 63 of the rectangular top wall 18. The triangular coupling hole 51 preferably is isosceles in form having sides 68 and 69 defining slant heights of A/ 2 and an apex angle equal to 90". In any case, the coupling hole 51 is constructed so as not to have a dimension larger than the dimension of the waveguide 52 in the corresponding direction. Although large coupling holes are desired for reasons of efficient coupling between the waveguide 52 and the cavity resonator 12, smaller coupling holes of other shapes could be employed. The design of the coupling hole is undertaken in accordance with standard impedance matching techniques, taking into consideration the impedance of the resonator 12, coupling hole 51, waveguide 52, and the microwave energy source. For heavy loads, it is desired to tightly couple to the resonator modes. Hence, a large coupling hole 51 would be employed, preferably triangular and covering the triangular segment 53. For light loads, looser coupling is desired to prevent undesirable reflections. In these cases, Smaller sized coupling holes would be employed. To facilitate impedance matching between the resonator 12 and any microwave energy source coupled thereto, a common adjustable impedance matching element 71 could be inserted in the waveguide path coupling the cavity resonator 12 to the energy source employed.

As explained hereinabove, to assure coupling to modes belonging to all of the subsets of resonator modes, the coupling hole 51 should 'be located in the top wall 18 so that the apex of the angle 0 defined by its sides 68 and 69 proximate the edge portions 54 and 55 of the top wall 18 converging to the wall corner 63 is not farther than A/2 /2 from the wall corner 63. Preferably, however, the coupling hole 51 .is located with their sides 68 and 69 at the edge portions 54 and 55 of the top wall 18. In the triangular shaped embodiment, the sides 68 and 69 are parallel to the edge portions 54 and 55 respectively, in the plane of the inside surface of the vertical end and side walls 13 and 17.

Coupling hole 51 is coupled in energy receiving relationship with a rectangular type waveguide 52 excited by a microwave energy source 74 to propagate energy in the dominant TE mode. The waveguide 52 is joined to the cavity resonator 12 by fastenng it to a mating rectangular waveguide flange 76 secured to the top wall 18 as by welding. The mating flange 76 and waveguide 52 are secured to the cavity resonator 12 so that the electric field of the electromagneitc field propagated by the dominant mode waveguide bisects the angle 0 defined at the wall corner 63.

If a TM mode waveguide is employed to excite the cavity resonator 12, an isosceles triangular shaped coupling hole 51, preferably of a size having slant heights of 71/ 4 and positioned as described hereinabove would be employed. As in the case when a TB mode waveguide is used, the coupling hole 51 could be smaller and of various shapes.

With particular reference to FIGURE 5, a waveguide flange 76 particularly suited for joining the waveguide 74 to the resonator 12 is shown in detail and is seen to comprise a waveguide transmission line portion 77, an apertured plate 78 defining the triangular coupling hole 51, and a corner bracket 79. The waveguide portion 77 has a flange portion 81 at one of its ends with flange bolt holes 82 thereabout for fastening to the waveguide 52. The opposite end of the waveguide portion 77 is secured to the plate 78 as by welding to enclose the triangular coupling hole 51. To assure that the coupling hole 51 functions as a coupling hole and not a short section of a waveguide, plate 78 is provided with a recess 83 for receiving the waveguide portion 77. The recess 83 reduces the thickness of the barrier 84 formed by the plate 78 between the waveguide portion 77 and the cavity resonator 12. A barrier wall thickness of inch is sufficient to insure that the coupling hole 51 will not function as a short section of a waveguide. The assembled waveguide portion 77 and plate 78 are fastened to the bracket 79 by suitable nuts and bolts 101 and 102 (see FIGURE 2) inserted through bolt holes 103 in plate 78 and bolt holes 104 in the lip portions 106 and 107 of the bracket 79. The bracket 79 is secured to the cavity resonator 12 by bolts 108 passing through holes 109 (see FIGURE 2) in the L-shaped corner member 111 of the bracket 79, to threadingly engage the resonator end wall 13 and side wall 17.

In one embodiment employing a 2.5 km. energy source 74 operating at about 2450 MHz, the slant heights of the coupling hole sides are 2.4 inches. A WR 340 rectangular waveguide 52 is employed to couple the source 74 to the cavity resonator 12. The guide dimensions of the WR 340 waveguide are a width of 3.4 inches and a height of 1.7 inches. The mating waveguide flange portion 77 has dimensions corresponding to the waveguide 52. Since the guide dimensions of the WR 340 waveguide 52 are larger than the dimensions of the triangular coupling hole 51, the obstacle 84 is formed at the coupling hole 51 by the plate 78. This obstacle insures coupling to the highest frequency modes of the resonator mode subsets that can exist in the cavity resonator 12.

To energize various modes of all of the resonator mode subsets, means 88 are also provided for shifting the frequencies at which the various modes occur so that sequentially and cyclically each mode pattern frequency coincides with the source frequency. In the embodiment illustrated in the figures, three mechanical type means 88 for mode stirring are employed of the type, for example, as described in the United States Application, Ser. No. 624,503 of Rexford E. Black, filed Mar. 20, 1967, entitled Disc Mode Stirrer and assigned to the assignee of this application. Each mode stirring means 88 includes a discshaped member 89 of conductive material having diametrically opposite chord segments 91 and 92 extending angularly from the plane of the disc member. The three disc-shaped members 89 are rotably mounted within the cavity resonator 12 to be rotated by a drive motor 93 located exteriorly of the resonator. As the disc-shaped members 89 are rotated, the electrical space of the enclosure 21 as seen by the electromagnetic field within the resonator changes. As the electrical space of the enclosure 21 changes, the frequencies of the resonator modes are caused to shift. As the frequencies of the resonator modes shift, dilferent ones will coincide with the frequency of the sources. In one embodiment stirrers 88 employing discs 89 measuring about eight inches in mode diameter with identical chord segments 91 and 92 having a chord length of about seven inches inclined away from the plane of the disc at 30 were located at the center of the upper half of the side wall 16, the center of the top wall 18, and the center of the upper quarter section of the end wall 14 proximate the side wall 16. This arrangement of mode stirrers 88 provide a time-averaged uniform electromagnetic field distribution in the zone about the converyor belt 22.

The average intensity of the electromagnetic field is uniform throughout most of the enclosure 21. However, in regions of the enclosure 21 measuring A/ 4 to M2 from the walls of the cavity resonator 12, the electric field intensity falls off rapidly to be zero at the walls. Therefore, to accomplish uniform heating of products, the conveyor belt 22 should be supported at least A/ 4 above the bottom wall 19 of the resonator 12 and, preferably at least 2. To support the conveyor belt 22 as it passes through the cavity resonator 12, a plurality of 2 inches by /2 inch slats 94, of polypropylene or other microwave transparent material, are supported by brackets 96 secured to the end walls 13 and 14 to extend longitudinally in the direction of the conveyor belt travel. The slats 94 are mounted about one foot above the bottom wall 19. To allow air to be circulated about the products transported through the cavity resonator 12, the slats 94 are spaced apart about /2 inch.

In many industrial applications an air flow is needed to maintain the preferred range of humidity at the products being heated. Since it is only necessary to direct the air flow over the surfaces of the products in large volume resonators, a panel 97 of polypropylene or other micro wave transparent material would be mounted just above the air ducts 42 and conveyor belt 22 by brackets 98 secured to the resonator walls to confine the air flow to the zone of the enclosure 21 through which the products pass. This makes more efiicient use of the air flow system.

In the embodiment of the present invention illustrated in the figures and described in detail hereinabove, energy was coupled into the cavity resonator 12 at a single location. However, additional noncoherent energy sources could be connected to deliver energy into the resonator 12 at additional wall corners or other locations of the resonator walls if desired. Delivering energy into the resonator 12 at a plurality of wall corners will facilitate, for example, introducing more energy into the resonator for heating materials and tailoring the time-averaged distribution of the electromagnetic field in the resonator as desired. The use of additional coupling holes may be employed when it is desired to generate non-uniform timeaveraged field distributions within the cavity resonator 12. The additional coupling holes and associated guide structures could be arranged to deliver energy into the cavity resonator 12 in the manner described in detail hereinabove. Or, they could be arranged to deliver energy into the cavity resonator 12 at wall corners thereof in the manner described in my copending application, Ser. No. 675,172, entitled Multimode Resonator for Heating Materials Excited At a Plurality of Corners of the Resonator Walls, and filled on Oct. 13, 1967.

While the present invention has been described in detail with respect to a particular embodiment, it is ap parent therefrom that numerous modifications and variations are possible within the spirit and scope of the invention. Particularly, the waveguide coupling of the multimode resonator to the microwave energy sources could be shortened in length so that the source would be mounted directly to the multimode resonator wall at the coupling hole with the output window commonly employed in the sources located at the coupling hole. Hence, the present invention is not to be limited except by the terms of the following claims:

1. Apparatus for heating materials with microwave energy contained in a selected time-averaged electromagnetic field distribution comprising a multimode microwave cavity resonator which is susceptible to being excited in a plurality of frequency dependent electromagnetic field mode patterns, said cavity having at least three conductive boundary walls which intersect at a junction defining a heating zone for subjecting the materials to microwave energy, a portion of one of the boundary walls having a coupling hole adjacent a corner thereof defined by wall edges intersecting at an angle at a junction of three intersecting walls of said cavity resonator for introducing microwave energy from a source into said cavity resonator through the plane of said wall, said coupling hole having a preponderance thereof located within a triangular wall zone measuring M 2 from the adjacent corner along the intersecting edges of said wall defining the adjacent corner and a line connecting the M2 points, where A is the free-space wavelength of the microwave energy coupled through said coupling hole, a single conductor wave-guide for delivering microwave energy from a source to said resonator through said coupling hole, said waveguide constructed and oriented at said coupling hole relative to said resonator so that the mode pattern in which energy is propagated by said waveguide structure has an electromagnetic field distribution including an electric field component in a direction of a line that divides the angle defined at the wall corner of said boundary wall defining said coupling hole.

2. The apparatus according to claim 1 including, means for shifting the frequency in which said resonator mode 0 patterns occur relative to the source frequency.

3. The apparatus according to claim 1 wherein said coupling hole is located entirely within said triangular wall zone.

4. The apparatus according to claim 1 wherein said coupling hole is triangular shaped with its apex at the adjacent wall corner.

, 5. The apparatus according to claim 1 further including a microwave energy source coupled to the waveguide to deliver microwave energy at a selected frequency to said resonator.

6. The apparatus according to claim 1 wherein said waveguide is a rectangular waveguide.

7. The apparatus according to claim 1 wherein the coupling hole is triangular having a base edge, side edges of a slant height of M2 defining an apex angle of said coupling hole located in the boundary wall with its apex at the adjacent wall corner; and said waveguide oined to the resonator so as to overlay the entire coupling hole. The apparatus according to claim 1 further including means for supporting the material within said resonator at least A/ 4 from the boundary walls of said resonator.

9. The apparatus accordng to claim 7 wherein said waveguide is joined to the resonator with a long side at the base edge of the coupling hole, and further including a wall portion at the end of the waveguide at the coupling hole to close the end of the waveguide not overlying the coupling hole.

10. The apparatus according to claim 2 wherein said means for shifting the frequency at which the resonator mode patterns occur relative to the source frequency is a mode stirrer located within said resonator.

11. The apparatus according to claim 5 further includg impedance matching means in circuit connection with said resonator, waveguide, and microwave energy source.

References Cited UNITED STATES PATENTS 2,648,760 8/1953 Hall et 'al. 219l0.55 3,365,562 1/1968 Jeppson 219-1055 FOREIGN PATENTS 1,188,745 3/1965 German.

JOSEPH V. TRUHE, Primary Examiner L. H. BENDER, Assistant Examiner U.S. Cl. X.R. 219-1061; 333-98 

